RE: For expressions and / operator in XPath 2.0 (Was: Re: [x

> Mike wrote:
> > In XPath 2.0 the answer is
> >
> >    sum(for $r in //rate return $r/@value * $r/@quantity)
>
> I feel sure that this is one of the more contentious aspects of XPath
> 2.0, but here goes...
>
> Why is it that the General Steps have to return nodes? If they didn't,
> you could do something like:
>
>   sum( //rate/(@value * @quantity) )
>
> I thought that the answer might be to do with the old set vs. sequence
> thing, and the fact that for backwards compatibility, the / operator
> has to union and sort the nodes.

That is indeed the reason. It was simply too messy to say "if the result is
a sequence of nodes, then they are returned in document order with
duplicates eliminated, if the result is a sequence of simple values, they
are returned as is, and if the result is a mixture of the two, then fire
breaks out." We needed to retain the "document order" semantics for
compatibility and for sensible behavior with recursive mixed content
document structures, and there wasn't a natural way of extending this to
values other than nodes.
>
> However, I can't work out why you can't just say that the / operator
> performs a union on node identity and a sort on document order; simple
> typed values have neither node identity nor document order, so
> sequences of simple typed values are left as they are.
>
It would mean, for example, that sum(XXX/@value) would not necessarily give
the same answer as sum(XXX/(@value+0)) (where XXX is an arbitrary
expression); though it would give the same answer 99% of the time, which
would make the exceptions even more surprising.

After months of agonising on this one, we just decided to keep things
simple: "/" eliminates duplicates and returns results in canonical order,
"for" retains duplicates and retains the ordering of the operand. I was sad
to lose "sum(//rate/(@value * @quantity))" but I'd rather make it an error
than have it return results that were difficult to explain to people.

Mike Kay


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